Method and apparatus for reference cell adjusting in a storage device

ABSTRACT

When adjusting the reference cells ( 11 ), the first reference unit ( 15 ) and the second reference unit ( 17 ) are used for a verify operation of the reference cells ( 11 ). The first reference unit ( 15 ) provides the lower limit current of the allowable current range, which is caused to flow by a nonvolatile transistor of the reference cells ( 11 ), and the second reference unit ( 17 ) provides the upper limit current thereof. An amplification signal REFO of the reference cells ( 11 ), which is outputted from a sense amplifier ( 19 ) is compared with amplification signals LO and HO responsive to the lower and upper limit currents outputted from sense amplifiers ( 21 ) and ( 23 ), and a verify operation is carried out to see whether or not the current caused to flow by the nonvolatile transistor is between the lower limit current and the upper limit current of the allowable current range.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation application of International Application No. PCT/JP2005/001334 filed Jan. 31, 2005, which was not published in English under PCT Article 21(2).

TECHNICAL FIELD

The present invention relates to a storage device provided with an accurate reference cell which detects the data state of a memory cell and a reference cell adjusting method for the storage device.

BACKGROUND Background Art

Some storage devices are provided with a reference cell for detecting the data state of a memory cell therein. For example, in a non-volatile storage device, the data state of a memory cell is detected by comparing the size of a current flowing in the memory cell and the current flowing in a reference cell during a verify operation of the memory cell in which an erase operation and/or a program operation are carried out, and in a read operation of data stored therein. It is necessary that the threshold voltage of a non-volatile transistor in the reference cell is adjusted to an adequate value for accurate verify operations.

Conventionally, reference cells have been adjusted by controlling the reference cells by means of a peripheral memory tester device. For example, the current caused to flow by a non-volatile transistor of the reference cell to which voltage is bias-applied is measured and the application bias is adjusted so that a specified current value is detected, wherein the threshold voltage, etc., are adjusted to an adequate state.

In recent years, storage devices have been proposed which is provided with a built-in self test (“BIST”). This is a storage device that is provided with an automatic diagnostic feature wherein testing of a large-scaled and highly integrated memory cell is independently carried out by a built-in control circuit. In the storage device which includes the BIST feature, a memory tester device is no longer required when testing memory cells. Therefore, it is necessary to automatically adjust the reference cell by a built-in control circuit when adjusting the reference circuit.

A method for programming a core cell is disclosed in Japanese Unexamined Patent Publication NO. 2003-323793 (“Patent Document 1”) wherein, when several phases of programs are provided and a selected program phase is passed, the process is caused to advance to the next program phase whose conditions are weakened and the core cell is programmed by repeating the process and verifying the final phase of the program.

Problems to be Solved by the Invention

The above-described Patent Document 1 attempts to resolve the problems of the prior art wherein repeated programming weakens the programming conditions phase by phase by not entering the core cell into an excessively programmed state by weakening the programming conditions in response to the programmed degree.

However, even if the programming condition is weakened phase by phase, the programming condition is not optimized in the respective phases, and, even if the programming condition is optimized, the programming status differs in respective core cells or between storage devices due to unevenness in production. Patent Document 1 discloses that verification of the programming is determined based on whether or not a specified threshold voltage is reached. Even if verification of the programming is detected by the final programming where the threshold voltage of the core cell exceeds the target threshold voltage, it is not possible to detect the degree by which the threshold voltage of the core cell exceeds the target threshold voltage and, therefore, the threshold voltage of the core cell for which programming is finished cannot be accurately controlled. The degree of overprogramming of the core cell can thus not be controlled, and such programming occurs, causing unevenness in the programming status of the core cell.

SUMMARY OF THE INVENTION

Means for Solving the Problems

The invention was developed in view of the above-described background art, and it is therefore an object of the invention to provide a storage device with a plurality of reference states including the lower limit state and the upper limit state when adjusting the reference data state of a reference cell, wherein the storage device is capable of accurately adjusting a reference data state of the reference cell by checking the reference data state of the reference cell, and to provide a method for adjusting the reference cell of the storage device.

A storage device in accordance with the present invention attempting to achieve the object stated above is provided with a reference cell to be referred to when determining the data state of a memory cell, the storage cell including a first reference unit for defining a lower limit state of a reference setting data state of the reference cell, a second reference unit for defining an upper limit state, and a comparator for comparing the reference data state of the reference cell to the lower limit state and the upper limit state.

With the storage device in accordance with the invention, when adjusting the reference data state of a reference cell to a reference setting data state, the lower limit state proposed by the first reference unit, the upper limit state proposed by the second reference unit, and the reference data state of the reference cell are compared with each other by a comparator.

Furthermore, a reference cell adjusting method of a storage device in accordance with the invention is provided with a reference cell to be referred to when determining a data state of a memory cell, the method comprising the steps of comparing a reference data state of the reference cell with a lower limit state of a reference setting data state of the reference cell and comparing the reference data state with an upper limit state of the reference setting data state.

In the reference cell adjusting method of the storage device in accordance with the invention, when adjusting the reference data state of a reference cell to the reference setting data state, the reference data state of the reference cell is compared with the lower limit state in the reference setting data state and, at the same time, is compared with the upper limit state in the reference setting data state.

Effects of the Invention

In accordance with the present invention, when adjusting the reference data state of the reference cell, both the lower limit state and the upper limit state of the reference setting data state are grasped and it is possible to check to see whether or not the reference data state has been adjusted between the upper limit state and the lower limit state.

Further, even in a case where the adjusting condition of the reference cell is not optimized and in a case where the states to be adjusted differ due to unevenness of production, the reference data state of the reference cell can be accurately adjusted to the reference setting data state and unevenness in the reference data state of the reference cell can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block for adjusting a reference cell in a nonvolatile storage device in accordance with an embodiment of the present invention;

FIG. 2 is a circuit diagram of a gate bias voltage control circuit in accordance with an embodiment of the present invention;

FIG. 3 is a view depicting a bias voltage outputted from the gate bias voltage control circuit in response to a bias control signal (REG(5:0)) in accordance with the embodiment of the present invention;

FIG. 4 is a sectional view of a nonvolatile transistor equipped in a reference cell in accordance with an embodiment of the present invention;

FIG. 5 is a view depicting applied states of a bias voltage to the reference cell in respective operation modes in accordance with an embodiment of the present invention;

FIG. 6 is a circuit diagram of the first and the second reference units in accordance with an embodiment of an present invention;

FIG. 7 is a circuit diagram of a sense amplifier in accordance with an embodiment of the present invention;

FIG. 8 is a view depicting the relationship between states of the reference cell and comparison result signals (REFOP, REFER) in accordance with an embodiment of the present invention;

FIG. 9 is a flowchart depicting a method for adjusting the reference cell in accordance with an embodiment of the present invention; and

FIG. 10 is a timing chart depicting adjustment operations of the reference cell in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a detailed description is given of a detailed embodiment with respect to a storage device in accordance with the invention and a reference cell adjusting method thereof based on FIG. 1 through FIG. 10.

In the embodiment of the present invention described herein, a description is given of an example of a nonvolatile storage device which is composed of memory cells provided with nonvolatile transistors having floating gates as a storage devices. That is, the storage device is of a type called called flash memory. Data is stored as a difference in the threshold voltage value of a memory cell in response to electric charge accumulated in a floating gate thereof by injection of charge into and discharge of charge from the floating gate. The accumulated state of charge, i.e., the data state of the floating gate, is compared with the reference setting data stored in a reference cell. The reference cell is provided with a nonvolatile transistor having the same composition as the nonvolatile transistor of the memory cells and electric charge is accumulated in advance in the floating gate.

The comparison operation is carried out by comparing the currents caused to flow by both of the nonvolatile transistors with each other when the same bias voltage is applied to the respective nonvolatile transistors of the memory cell and the reference cell. This is called a verify operation. There are reference cells used for verify operations for erasing, programming, soft programming, correction of over erasing, reading, etc. Reference cells having reference setting data states exclusive for the respective verify operations stored therein are provided to carry out the verify operations.

The circuit block diagram of FIG. 1 depicts a circuit portion of the nonvolatile storage device for adjusting the reference cells. Reference cells 11 are provided for an erase state, a program state, a soft program state, a correction state of the over erase, a read state (verify operation), etc., respectively, as described above. Or, where the storage device is composed so as to include a plurality of banks, it is considered that the reference cells 11 are provided for each of the banks. The current caused to flow by the nonvolatile transistor (Refer to FIG. 4) of the reference cell in response to application of a specified voltage bias by a verify operation is compared with the current caused to flow by the nonvolatile transistor of the memory cell (not illustrated) to which the same bias is applied, whereby the data state in the memory cell can be detected.

The reference cells 11 are selected by selectors 9 and 13 in order to adjust the respective reference cells 11. A selection operation of the selectors 9 and 13 is carried out in response to a selection signal SEL outputted by a control circuit 1 described later. The reference data state is adjusted for each of the selected reference cells 11. Adjustment of the reference data states is carried out by accumulating electric charge into the floating gate of the nonvolatile transistor equipped in the reference cells 11 as described later.

The control circuit 1 to control the adjustment operation outputs various types of control signals and simultaneously receives feedback signals of the adjustment result. The control circuit 1 adjusts the reference cells 11 to the reference setting data state in compliance with the specified adjustment procedure described later in FIG. 9. Herein, the control circuit 1 can be, for example, a BIST control circuit wherein an adjustment feature of reference cells to the reference data states is included in the self diagnosis feature based on the BIST. Accordingly, prior to testing a memory cell by the BIST, the reference cells 11 to detect the data state in the memory cell can be adjusted to the reference setting data state.

The control circuit 1 controls a gate bias voltage control circuit 3, a positive voltage generating circuit 5, and a negative voltage generating circuit 7 in addition to the selectors 9 and 13. Also, it controls sense amplifiers 19, 21 and 23 for verification.

The gate bias voltage control circuit 3 outputs an adjustment voltage bias (during a program operation) or a specified voltage bias (during a verify operation) which is applied to the pre-gate terminal REF_G of the reference cells 11 when programming or carrying out verification (reading). The gate bias voltage control circuit 3 is activated by an enable signal EN from the control circuit 1. And, the adjustment voltage bias or specified voltage bias is controlled to be variable in response to the bias control signal REG(5:0) having a six-bit composition.

The positive voltage generating circuit 5 supplies a bias of positive voltage to the pre-drain terminal REF_D of the reference cells 11 and the pre-well terminal REF_WL thereof. The positive voltage generating circuit 5 supplies the bias of positive voltage to the pre-well terminal REF_WL in response to an erase operation instruction REF_ER from the control circuit 1 and supplies the bias of positive voltage to the pre-drain terminal REF_D in response to the program operation instruction REF_PGM from the control circuit 1.

The negative voltage generating circuit 7 supplies a bias of negative voltage to the negative power pre-source terminal REF_N of the reference cells 11 in response to the erase operation instruction REF_ER from the control circuit 1. The negative power pre-source terminal REF_N is connected to the gate terminals (G) of the reference cells by a control circuit (not illustrated). When an erase operation is executed, the gate terminals (G) of the reference cells 11 are biased to a negative voltage.

The pre-gate terminal REF_G connected to the gate bias voltage control circuit 3, the pre-drain terminal REF_D and the pre-well terminal REF_WL connected to the positive voltage generating circuit 5, and the negative power pre-source terminal REF_N connected to the negative voltage generating circuit 7 are, respectively, connected to the gate terminals (G), drain terminals (D) and well terminals (WL) of the reference cells 11 as selected by the selector 9.

The reference cells 11 which are selected to apply a voltage bias thereto by the selector 9 are connected to the data bus REF_DB for the reference cell via the selector 13. A data current read from the reference cells 11 to which a bias is applied is caused to flow to the data bus REF_DB via the selector 13 when a verify operation is executed. The data bus REF_DB is connected to the input terminal (I) of the sense amplifier 19. The sense amplifier 19 amplifies the data current in response to the verification control signal REF_V outputted from the control circuit 1 and inputted into the enable terminal (EN). The amplified data current is converted in terms of voltage and is outputted as an amplified signal REFO.

In detail, the data bus REF_DB is connected to the drain terminal of the selected nonvolatile transistor(s) in the reference cells 11. As described later, the input terminal (I) of the sense amplifier 19 is maintained at a verify reading voltage, applying the verify reading voltage to the drain terminal of the selected nonvolatile transistor of the reference cells 11. The current flowing into the data bus REF_DB is determined in response to an accumulated charge volume equivalent to the reference data state stored in the nonvolatile transistor together with the specified voltage bias outputted from the gate bias voltage control circuit 3, biased to the gate terminal.

The first reference unit 15 and the second reference unit 17 are used for verify operations of the reference cells 11 and detect whether or not the reference data state stored in the reference cells 11 is in the reference setting data state. The first reference unit 15 provides the lower limit state of the reference setting data state and the second reference unit 17 provides the upper limit state of the reference setting data state. Herein, the reference setting data state of the reference cells 11 is defined as an allowable current range caused to flow in the nonvolatile transistor when a specified voltage bias and a verify reading voltage are applied to a nonvolatile transistor of the reference cells 11. The lower limit state provided by the first reference unit 15 provides the lower limit current of the allowable current range and the upper limit state provided by the second reference unit 17 provides the upper limit current of the allowable current range. Both the lower limit current and the upper limit current are inputted into the input terminals (I) of the sense amplifiers 21 and 23 via the lower limit current bus DBL and the upper limit current bus DBH, respectively. In the sense amplifiers 21 and 23, verification control signal REF_V outputted from the control circuit 1 and inputted into the sense amplification 19 is inputted into the enable terminals (EN). The lower limit and the upper limit currents inputted into the input terminals (I) are amplified in response to the verification control signal REF_V. The lower limit and the upper limit currents thus amplified are converted in terms of voltage and are outputted as amplification signals LO and HO.

An amplification signal REFO showing the reference setting data state of the reference cells 11, which is outputted from the sense amplifier 19, is compared with the amplification signal LO responsive to the lower limit current and the amplification signal HO responsive to the upper limit current by inputting the amplification signal REFO into an inverting input terminal of the comparator 25 and into a non-inverting input terminal of the comparator 27 and inputting the amplification signals LO and HO, respectively, into the non-inverting input terminal of the comparator 25 and into the inverting input terminal of the comparator 27.

Comparison result signals REFOP and REFER of the comparators 25 and 27 are fed back to the control circuit 1. The control circuit 1 carries out an adjustment operation of the reference cells 11 in response to the comparison result signals REFOP and REFER.

FIG. 2 is a circuit of the gate bias voltage control circuit 3 in accordance with an embodiment of the present invention. The gate bias voltage control circuit 3 is provided with a ladder resistance network RA, a level shifter LS connected to the respective input terminals P0 through P5 of the ladder resistance network RA, NMOS transistors MN1 and MN2 by which both terminals of the ladder resistance network RA are connected to the grounding potential, and a buffer circuit BUF for receiving an output terminal X of the ladder resistance network RA. In addition, an additional level shifter LS shifts the level of the enable signal EN and outputs the same to the NMOS transistors MN1 and MN2.

The level shifter LS converts the high-level voltage of the bias control signal REG(5:0) and enable signal EN from the power source voltage supplied to the control circuit 1 to the high-level voltage VH in terms of voltage level. The converted six-bit signal is inputted into the input terminals P0 through P5 of the ladder resistance network RA.

In the ladder resistance network RA, the input terminals P0 through P5 are connected via resistance elements having a resistance value 2R to respective contact points, to which resistance elements having a resistance value R are connected in series. The NMOS transistors MN1 and MN2 are connected via resistance elements having a resistance value 2R to both ends of the contact points to which the resistance elements of the resistance value R are connected in series. The other ends of the NMOS transistors are connected to the grounding potential as the source terminals. Also, the contact point to which the input terminal P5 is connected via the resistance element having the resistance value 2R is an output terminal X. Herein, with respect to the bias control signal REG(5:0), the bias control signal REG(0) is LSB and the bias control signal REG(5) is MSB.

FIG. 3 depicts bias voltages V (REF_G) which are controlled in response to the bias control signal REG(5:0) and outputted from the gate bias voltage control circuit 3 in accordance with the embodiment of the present invention. As clarified in the graph, the bias voltage V (REF_G) increases and decreases in compliance with an increase and a decrease in the bias control signal REG(5:0) being the six-bit signal.

When all the bias control signals REG(5:0) are at a low level, all of the input terminals P0 through P5 of the ladder resistance network RA in FIG. 2 are at the grounding potential (0V). At this time, the NMOS transistors MN1 and MN2 are conductive and both terminals of the resistance elements connected in series in the ladder resistance network RA are at the grounding potential (0V). Therefore, the grounding potential is outputted as the output terminal X. 0V is outputted, which is the minimum bias voltage V(REF_G).

When all of the bias control signals REF(5:0) are at a high level, all of the input terminals P0 through P5 of the ladder resistance network RA in FIG. 2 are at the high-level voltage VH. On the other hand, both of the terminals of the resistance elements connected in series are at the grounding potential (0V). Based on the operation of electric circuit theory, the voltage value at the output terminal X is roughly two-thirds the high-level voltage VH (that is, (⅔)VH).

In respective phases at which the bias control signals REG(5:0) reach the high level following the case where all the bias control signals REG(5:0) are at a low level, voltages which are the (⅔)VH voltage divided are outputted from the output terminal X. The voltage dividing value with respect to the respective bias control signals REG(5:0) is as shown in FIG. 3.

FIG. 4 depicts a nonvolatile transistor of the reference cells 11 in accordance with an embodiment of the present invention. The nonvolatile transistor is provided with N+ diffusion areas in the P well forming a source terminal (S) and a drain terminal (D). A floating gate (FG) and, further, a control gate (CG) are stacked over a channel between the source terminal (S) and the drain terminal (D) with a gate oxidization membrane (not illustrated) placed therebetween. The control gate (CG) is the gate terminal (G). Also, the P well is connected to a well terminal (WL) via a P+ diffusion layer.

FIG. 5 depicts bias conditions applied onto the respective terminals during respective operations. The erase operation shows a case in which a nonvolatile transistor is erased. A negative voltage V (REF_N) is applied to the gate terminal (G), which is the negative voltage generated by the negative voltage generating circuit 7 (FIG. 1). The source terminal (S) and the drain terminal (D) are placed into a floating state. And a positive voltage V (REF_WL) is applied to the well terminal (WL), which is the positive voltage generated by the positive voltage generating circuit 5 (FIG. 1).

During the program operation, a positive voltage V(REF_G) is applied to the gate terminal (G), which is the positive voltage generated by the gate bias voltage control circuit 3 (FIG. 1). The source terminal (S) is held at the grounding potential, and a positive voltage V(REF_D) is applied to the drain terminal (D), which is the positive voltage generated by the positive voltage generating circuit 5 (FIG. 1). The well terminal (WL) is also held at the grounding potential. The positive voltage V(REF_G) of this case is an adjustment voltage bias outputted from the gate bias voltage control circuit 3 and is variably controlled in response to the bias control signal REF (5:0). It is possible to control the voltage stress relating to the program operation with the variable gate bias voltage during the program operation of the reference cells 11.

During the verify operation, a positive voltage V(RD) is applied to the gate terminal (G), which is a positive voltage generated by the gate bias voltage control circuit 3 (FIG. 1). The source terminal (S) is held at the grounding potential, and the positive voltage V(REF_DB) is applied to the drain terminal (D), which is a verify reading voltage applied to the sense amplifier (FIG. 7) described later. The well terminal (WL) is also held at the grounding potential. Herein, the positive voltage V(RD) is a specified voltage bias outputted from the gate bias voltage control circuit 3 controlled in response to a specified bias control signal REG(5:0).

FIG. 6 is a circuit of the first reference unit 15 and the second reference unit 17 in accordance with an embodiment of the present invention. The circuit includes a resistance element therein. One end of the resistance element is connected to the grounding potential and the other terminal is connected to the lower limit current bus DBL or the upper limit current bus DBH. As described later, in order to keep the lower limit current bus DBL and the upper limit current bus DBH at a specified voltage by the sense amplifiers 21 and 23, the current values flowing into the lower limit current bus DBL and the upper limit current bus DBH are calculated in response to the resistance values RL and RH of the resistance element. The resistance value RH of the resistance element of the second reference unit 17 which outputs the upper limit current is set to be lower than the resistance value RL of the resistance element of the first reference unit 15 which outputs the lower limit current (i.e., RH<RL).

FIG. 7 is a circuit of the sense amplifiers 19, 21, and 23 in accordance with an embodiment of the present invention. The input terminal (I) is connected to the gate terminal of an NMOS transistor MN4 and the enable terminal (EN) is connected to the input terminal of the inverter gate I1. The output terminal of the inverter gate I1 is connected to the gate terminals of an NMOS transistor MN3 and PMOS transistors MP1 and MP2. The source terminals of the PMOS transistors MP1 and MP2 are connected to the power source voltage VCC and the source terminals of the NMOS transistor MN3 and MN4 are connected to the grounding potential.

The drain terminal of the PMOS transistor MP1 is connected to the output terminal (O) via the resistance element R1 and connected to the drain terminal of the NMOS transistor MN5. The source terminal of the NMOS transistor MN5 is connected to the input terminal (I) and the drain terminal of the NMOS transistor MN3, as well as connected to the gate terminal of the NMOS transistor MN4.

The drain terminal of the PMOS transistor MP2 is connected to the gate terminal of the NMOS transistor MN5 and the drain terminal of the NMOS transistor MN4 via the NMOS transistor MN6. The gate terminal of the NMOS transistor MN6 is connected to the power source voltage VCC.

A description is next given of operation of the sense amplifiers. An amplification operation is commenced by inputting a high-level verification control signal REF_V into the enable terminal (EN). A low-level signal is supplied into the gate terminals of the NMOS transistor MN3 and PMOS transistors MP1 and MP2 via the inverter gate I1, wherein the NMOS transistor MN3 is placed into a non-conductive state and the PMOS transistors MP1 and MP2 are placed into conductive states.

With the PMOS transistor MP2 in a conductive state, a high potential voltage level is applied to the gate terminal of the NMOS transistor MN5 via the NMOS transistor MN6, thereby placing the NMOS transistor MN5 into a conductive state. A current channel is formed from the PMOS transistor MP1, which is in a conductive state, to the reference cells 11 or the first reference unit 15 and the second reference unit 17 via the resistance element R1 and the NMOS transistor MN5, passing through the input terminal (I) and data bus REF_DB or the lower limit and upper limit current bus DBL/DBH. The current value flowing in the current channel is determined in compliance with the resistance value of the resistance element R1, the conductive state of the NMOS transistor MN5 and the nonvolatile transistor of the reference cells 11 or the resistance elements of the first reference unit 15 and the second reference unit 17.

The current value is controlled to be an almost constant current value by the input terminal (I) being controlled at an almost constant voltage level by the feedback configuration composed of the NMOS transistors MN4 and MN5. That is, as the current capacity of the NMOS transistor MN5 increases, a greater current is caused to flow from the input terminal (I) to the reference cells 11 or the first reference unit 15 and the second reference unit 17, wherein the voltage level of the input terminal (I) increases. An increase in the voltage level of the input terminal (I) results in an increase in the voltage level of the gate terminal of the NMOS transistor MN4, thereby expanding the current capacity of the NMOS transistor MN4. An increase in the current capacity of the NMOS transistor MN4 causes the voltage level of the gate terminal of the NMOS transistor MN5 to drop, and the current capacity of the NMOS transistor MN5 is decreased. On the other hand, if the current capacity of the NMOS transistor MN5 decreases, the voltage level of the input terminal (I) decreases. The voltage level of the input terminal (I) dropping causes the current capacity of the NMOS transistor MN4 to decrease. Therefore, the voltage level of the gate terminal of the NMOS transistor MN5 increases and the current capacity of the NMOS transistor MN5 increases.

The feedback configuration composed of the NMOS transistors MN4 and MN5 is one example of the voltage application portion in accordance with the present invention, which is an example of a case where the voltage V(I) of the input terminal (I) controlled at an almost constant voltage level is applied to one end of the drain terminal or resistance element of a memory cell when carrying out verification.

Accordingly, where the charge volume accumulated in the nonvolatile transistor of the reference cells 11 is increased, or where the first reference unit 15 having a greater resistance value RL (i.e., greater than RH) is connected, the current volume flowing with respect to the verify reading voltage V(I) applied to the input terminal (I) is limited. The voltage volume drop from the PMOS transistor MP1 via the resistance element R1 is decreased and a high voltage level signal is outputted from the output terminal (O).

Also, where the charge volume accumulated in the nonvolatile transistor of the reference cells 11 is decreased or where the second reference unit 17 having a smaller resistance value RH (i.e., less than RL) is connected, the current volume flowing with respect to the verify reading voltage V(I) applied to the input terminal (I) is increased. And, consequently, the voltage volume drop from the PMOS transistor MP1 via the resistance element R1 is increased and a low voltage level signal is outputted from the output terminal (O).

A pause is brought about in response to a low-level verification control signal REF_V inputted into the enable terminal (EN). A high-level signal is thus supplied into the gate terminals of the NMOS transistor MN3 and PMOS transistors MP1 and MP2 via the inverter gate I1. The NMOS transistor MN3 is placed into a conductive state and the PMOS transistors MP1 and MP2 are placed into a non-conductive state.

By comparing amplification signals REFO, LO and HO outputted from the output terminal (O) of the respective sense amplifiers 19, 21, and 23 by means of the comparators 25 and 27 (FIG. 1), it is possible to check the state of the reference cells 11 with respect to the respective states of the first reference unit 15 and the second reference unit 17.

The states in accordance with the embodiment of the present invention are depicted in FIG. 8. A graph in which the voltage level V (REF_G) of the pre-gate terminal REF_G is placed as the abscissa and the current I(REF_DB) flowing in the data bus REF_DB when verifying is placed as the ordinate indicates the characteristics of a nonvolatile transistor of the reference cells 11. That is, the graph shows three states: an erase state ER where a current exceeding the upper limit current I (DBH) set by the second reference unit 17 flows with respect to the verify reading voltage V(I); a program state PGM where a current between the lower limit current I(DBL) and the upper limit current I(DBH), which are set by the first reference unit 15 and the second reference unit 17, flows; and an over-program state O_PGM where a current being less than the lower limit current I(DBL) set by the first reference unit 15 flows.

The current flowing in the reference cells 11 with respect to the verify reading voltage V(I) of the sense amplifiers 19, 21, and 23 is compared with the lower limit current I(DBL) and the upper limit current I(DBH). In the erase state ER, a greater current in comparison with the upper limit current I(DBH) flows. The voltage thereof is converted by the sense amplifiers 19 and 23 to invert the relationship in size, wherein the amplification signal REFO is placed into a low voltage level in comparison with the amplification signal HO and the comparison result signal REFER is placed into a low level by the comparator 27. In the over-program state O_PGM, a smaller current is caused to flow as compared to the lower limit current I(DBL). The voltage is converted by the sense amplifiers 19 and 21 to invert the relationship of the currents, wherein the amplification signal REFO is placed into a high voltage level in comparison with the amplification signal LO. And, the comparison result signal REFOP is placed into a low level by the comparator 25. In the program state PGM, an intermediate current between the lower limit current I(DBL) and the upper limit current I(DBH) flows, wherein the comparison result signals REFER and REFOP are both placed in a high level by the comparators 25 and 27.

Thus, FIG. 8 depicts the states where the relationship in size is inverted with the voltage converted by the sense amplifiers 19, 21 and 23, the abscissa of which indicates the voltage value V(REFO) of the amplification signal REFO and the ordinate of which indicates the current I (REF_DB) flowing in the data bus REF_DB.

Based thereon, it is detected whether or not the reference setting data state of the reference cells 11 is adjusted to be within an allowable current range, which is between the lower limit current I(DBL) and the upper limit current I(DBH).

FIG. 9 depicts a flowchart of a method for adjusting the reference cells 11 in accordance with an embodiment of the present invention. First, the process is initialized (S1), thereby initializing the repetition variable RETNO by which the maximum number of times of repetition in the adjustment routine is counted (that is, RETNO=0). Next, an erase operation mode is set as an operation mode (S2). When adjusting the reference cells 11, an erase operation of the nonvolatile transistor is carried out and the charge of the floating gate is discharged.

When commencing application of an actual adjustment voltage bias after the erase operation mode is set, it is preferable that the reference data state of the reference cells 11 is checked. A verify operation is carried out in connection with the second reference unit 17 (S4). Where the current flowing in the reference cells 11 is smaller than the upper limit current I (DBH) (S4:F), it is judged that the nonvolatile transistor of the reference cells 11 does not reach the erase state, wherein voltage stress is applied (S3). The voltage stress applied herein is a stress in the erase operation as depicted in FIG. 5. Where the current flowing into the reference cells 11 is larger than the upper limit current I (DBH) (S4:T), it is judged that the nonvolatile transistor of the reference cell is in an erase state. Since, in this phase, the erase operation mode is established, and the mode is not the program operation mode (S5:F), the program operation mode is set as the next operation mode (S12), and the process returns to Step (S4).

After being set to the program operation mode, a verify operation is carried out with respect to the second reference unit 17 in the step (S4), and while the current flowing into the reference cells 11 is larger than the upper limit current I(DBH) (S4:F), it is judged that the nonvolatile transistor of the reference cells 11 does not reach the upper limit current in an allowable current range in the program state, wherein voltage stress is applied (S3). When it is detected that the current flowing into the reference cells 11 is smaller than the upper limit current I (DBH) (S4:T), since the mode is set to the program operation mode (S5:T), the next verify operation is carried out.

The next verify operation is carried out with respect to the first reference unit 15 (S6). If the current flowing into the reference cells 11 is larger than the lower limit current I(DBL) (S6:T), it is confirmed that the nonvolatile transistor of the reference cell is in an allowable current range in the program state, and the adjustment of the reference cells 11 is finished.

If the current flowing into the reference cells 11 is smaller than the lower limit current I(DBL) (S6:F), the nonvolatile transistor of the reference cells 11 is operating less than the lower limit current of the allowable current range in the program state. This indicates an over-program state. In this case, since it has been determined that the state is not adjusted to a normal program state, the process of re-adjustment is carried out in compliance with the procedure of step (S7) and the following steps.

In Step (S7), the operation mode is set to an erase operation mode. Next, the bias control signal REG(5:0) outputted by the control circuit 1 is revised to a value smaller by one bit (S8). One is added to the value of the repetition variable RETNO (S9). Then, it is confirmed whether the number is less than the maximum number of times of repetition (S10:F). If so, the process shifts back to Step (S4). Since the erase operation mode is set, an erase operation is carried out by the verify operation (S4) and voltage stress is applied (S3). After the erase operation is completed, the program operation mode is established in compliance with the step (S12), wherein the voltage stress (S3) for program and the verify operation (S4, S6) are carried out.

The above-described readjustment step is carried out within the maximum number of times of repetition (S10:F) until the reference cells 11 flow current within the allowable current range when carrying out a verify operation. When the number of times of re-adjustment reaches the maximum number of times of repetition (S10:T), an error process is carried out (S11). An error is outputted, which indicates the adjustment of the reference cells 11 has not been completed.

FIG. 10 is a timing waveform depicting an adjustment operation of the reference cells 11 in accordance with the embodiment of the present invention. Herein, it is assumed that the storage device is provided with five banks and the respective banks include reference cells (1) through (5). The drawing depicts operation waveforms for carrying out adjustment for the respective reference cells (1) through (5). When commencing the adjustment operation, an erase operation is executed in response to a high-level erase operation instruction REF_ER from the control circuit 1 as an initialize operation. Application of voltage stress and a subsequent verify operation are carried out for each of the reference cells 11 by the selectors 9 and 13.

The verify operation is carried out in response to a high level of the verification control signal REF_V. The gate terminal is biased by a specified voltage bias controlled by the bias control signal REG(5:0) which becomes 35 in a decimal system value, and a verify operation is executed. The comparison result signal REFER is placed in a low level whenever it is confirmed by the verify operation that the selected reference cells 11 are in an erase state. The erase operation is carried out one after another with the reference cells 11 changed by selection. Also, the voltage stress to the gate terminal in the erase operation is a negative voltage. Since a negative voltage generated by the negative voltage generating circuit 7 is applied, 0 is outputted in a decimal system value for the bias control signal REG(5:0). In addition, although not depicted in FIG. 10, the gate bias voltage control circuit 3 is entered into a disable state by a low-level enable signal EN.

After the erase operation is completed for all of the reference cells (1) through (5), a high-level program operation instruction REF_PGM is outputted from the control circuit 1, shifting the process to the program operation. As in the erase operation, the program operation is carried out for each of the reference cells 11 by the selectors 9 and 13. Application of a voltage stress and a subsequent verify operation are carried out for each of the reference cells 11 by the selectors 9 and 13.

The voltage stress is applied with the gate terminal biased by an adjustment voltage bias controlled by the bias control signal REG(5:0) which becomes 29 in a decimal system value. After the adjustment voltage bias is applied for a specified period of time, the reference cells 11 are selected one after another and a program operation is executed with the verify operation being finished.

The verify operation is carried out in response to the high level of the verification control signal REF_V, following application of the adjustment voltage bias. The gate terminal is biased by a specified voltage bias controlled by the bias control signal REG(5:0) which becomes 35 in a decimal system value, and the verify operation is carried out. Whenever the program state is confirmed by a verify operation with respect to a selected one of the reference cells 11, both the comparison result signals REFER and REFOP are entered into a high level, whereby it is confirmed that the reference cells 11 have been programmed within the allowable current range. Reference cells 11 are selected one after another in response to a normal program state being confirmed, and the program operation is executed (in FIG. 10, reference cells (1) through (3)).

Herein, as the program state of the reference cells 11 is entered into an over-program state, the current flowing from the reference cells 11 is has a smaller current value than the lower limit current I(DBL), and the comparison result signal REFOP is placed in a low level.

The control circuit 1 that has received the high-level comparison result signal REFER and the low-level comparison result signal REFOP shifts to a process of re-adjustment. An erase operation is carried out with the erase operation instruction REF_ER kept at the high level and the program operation instruction REF_PGM kept at the low level, and the reference cell is initialized. Although the program operation is carried out after the erase operation, in this case, the control circuit 1 outputs a value subtracted by 1 to a value 29 in the last program operation with the value of the bias control signal REG(5:0) made into 28 in a decimal system value. The adjustment voltage bias outputted from the gate bias voltage control circuit 3 is made into a smaller voltage value by an amount of (⅔)VH×( 1/96) less than the last voltage bias as depicted in FIG. 3. The voltage stress to the reference cells 11 is mitigated during the program operation, wherein it is possible to further smoothly program the reference cells 11 and excessive stress application which causes overprogram can be prevented. Simultaneously, 1 is added to the variable RETNO to control the number of times of repetition of the adjustment routine. Further, the variable RETNO is not specifically described in terms of the circuits, which is a variable processed in the control circuit 1. Also, if a counter circuit or the like is provided in the control circuit 1 or separately therefrom, it is possible to control the number of times of repetition of the adjustment routine.

As described, in order to adjust the reference data state of a reference cell, a program operation is carried out for a nonvolatile transistor of the reference cell and a charge amount accumulated in the floating gate (FG) is adjusted. At this time, in accordance with the present embodiment, it is possible to confirm the lower limit state and the upper limit state of the set reference data state in the reference cell, that is, to confirm whether or not the current flowing I(REF_DB) when a specified voltage bias is applied to the nonvolatile transistor of the reference cell has been adjusted in the lower limit current I(DGL) and the upper limit current I(DBH) of the allowable current range. Where the charge in the floating gate (FG) is accumulated by executing a program operation after the erase operation is initialized, since the current I(REF_DB) flowing from the reference cell is detected in relation to the upper limit current I(DBH) in addition to the lower limit current (DBL), it is possible to detect not only an insufficient program state but also an excessive program state, that is, an over-program state.

Where the adjustment condition of the reference cells is not optimized and where the adjusted condition differs due to unevenness in production of the reference cells, it is possible to accurately adjust the operational current of the reference cells to within the allowable current range, and it is possible to reduce unevenness of the reference cells.

Also, as a result of a program operation, it is possible to easily detect that the reference cells have been subjected to an over-program state and, at the same time, it is possible to supply an accurate adjustment voltage bias to the reference cells when carrying out re-adjustment when the over-program state is detected. That is, since the voltage value of the adjustment voltage bias outputted from the gate bias voltage control circuit 3 can be variably controlled in response to the value of the bias control signal REG(5:0) of six bits, the adjustment voltage bias can be reduced at the ratio of a step obtained by dividing the adjustment voltage bias into the maximum value of 64 (i.e., 2⁶) by subtracting 1 from the decimal system value of the bias control signal REG(5:0) in a case of an over-program state. That is, it is possible to weaken the voltage stress during the program operation by a 1/64-divided step, and it is possible to prevent an over-program state resulting from excessive stress application from occurring.

In addition, the invention is not limited to the above-described embodiment and the invention can be subjected to various improvements and modifications within the scope of the claims below and their equivalents without departing from the spirit of the invention.

For example, in the operation timing waveform depicted in FIG. 10, a description was given of a case where, when an over-program state is brought about as a result of a program operation, the bit value of the bias control signal REG(5:0) is subtracted from 29 to 28 by 1 in a decimal system value. However, the invention is not limited thereto. It is possible to adequately determine a ratio of subtraction other than that disclosed.

Also, in FIG. 10, a description was given of a case where all of the reference cells (1) through (5) are processed again for re-adjustment. However, the invention is not limited thereto; the re-adjustment may be carried out only for the reference cell for which an over-program state is detected.

In addition, a description was given of a case where, when confirming the reference data state of a reference cell, the current caused to flow by the reference cell during a verify operation is compared with the lower limit current and the upper limit current. However, such a construction may be employed in which the current is compared with an intermediate current in addition to the lower limit current and the upper limit current, whereby it is possible to further accurately grasp the state of the reference cell during the program operation.

Furthermore, a description was given of a case of providing a plurality of values for the lower limit current and the upper limit current during the program operation. However, it is considered that, during an erase operation that is an initialize operation prior to a program operation, a verify operation can be carried out for a plurality of reference current values including an intermediate current in addition to the lower limit current and the upper limit current. Thereby, the reference cell state can be accurately controlled during the erase operation, and it is possible to prevent unevenness during the erase state, such as over-erasing. 

1. A storage device having a reference cell to be referred to when determining the data state of a memory cell, the storage device comprising: a first reference unit for defining a lower limit state of a reference setting data state of the reference cell; a second reference unit for defining an upper limit state of the reference setting data state; and a comparator for comparing a reference data state of the reference cell to the lower limit state and the upper limit state.
 2. The storage device of claim 1 further comprising: a control unit for adjusting the reference data state in response to the reference setting data state, wherein the control unit initializes the reference cell when the comparator determines that the reference cell is adjusted below the lower limit state or above the upper limit state, relaxes adjustment bias applied to the reference cell, and adjusts the reference data state again.
 3. The storage device of claim 1 wherein: the memory cell and the reference cell comprise nonvolatile transistors; the reference setting data state is generated in an allowable current range of current flowing in the nonvolatile transistors when a specified voltage bias is applied; and the lower limit and the upper limit states are a lower limit current and an upper limit current within the allowable current range.
 4. The storage device of claim 3 wherein: the first reference unit comprises a first resistance element for generating the lower limit current; and the second reference unit comprises a second resistance element for generating the upper limit current.
 5. The storage device of claim 4 wherein: each of the first and second resistance element is coupled to a voltage application unit for applying a verify reading voltage.
 6. The storage device of claim 3 further comprising: a bias voltage control unit operating in response to a bias control signal, the bias control signal composed of a plurality of bits for issuing the specified voltage bias and for issuing an adjustment voltage bias applied to the nonvolatile transistor when adjusting the reference data state.
 7. The storage device of claim 6 wherein: the bias voltage control unit includes a ladder resistance network.
 8. The storage device of claim 6 further comprising: a control unit coupled to the bias voltage control unit for issuing the bias control signal and for adjusting the current flow when the specified voltage bias is applied to the nonvolatile transistor within the allowable current range, wherein the control unit initializes the nonvolatile transistor when the comparator determines that the nonvolatile transistor has been adjusted outside the allowable current range, updates the bias control signal to reduce the voltage value of the adjustment voltage bias, and adjusts the current flow again.
 9. The storage device of claim 8 wherein: the control unit adjusts the current flow of the nonvolatile transistor within the allowable current range by performing an erase operation of the nonvolatile transistor and thereafter performing a program operation thereof.
 10. The storage device of claim 9 wherein: the control unit determines that the erase operation is completed when the current flow from the nonvolatile transistor when applying the specified voltage bias is greater than the upper limit current, and wherein the control unit determines that the program operation is completed when the current flow from the nonvolatile transistor when applying the specified voltage bias is between the lower limit current and the upper limit current.
 11. The storage device of claim 9 wherein: the control unit determines that the program operation is in the over-program state when the current flow from the nonvolatile transistor when the specified voltage bias is applied is smaller than the lower limit current, and wherein the control unit in response thereto decreases the program current after the erase operation, and then performs the program operation again.
 12. A reference cell adjusting method of a storage device having a reference cell to be referred to when determining a data state of a memory cell, the method comprising the steps of: comparing a reference data state of the reference cell with a lower limit state of a reference setting data state of the reference cell; and comparing the reference data state with an upper limit state of the reference setting data state.
 13. The reference cell adjusting method of the storage device of claim 12 further comprising, when adjusting the reference cell in the reference setting data state, the steps of: applying an adjustment bias to the reference cell; reducing the adjustment bias when the reference data state adjusted at the step of applying the adjustment bias is outside the state between the lower limit state and the upper limit state as a result of the comparing step; initializing the reference cell when the reference data state adjusted at the step of applying the adjustment bias is outside the reference setting data state as a result of the comparing step; and applying the reduced adjustment bias to the reference cell after the initializing step.
 14. The reference cell adjusting method of the storage device of claim 12 wherein: the memory cell and the reference cell comprise nonvolatile transistors; the reference setting data state is determined in an allowable current range of current flow from the nonvolatile transistors when a specified voltage bias is applied to a gate terminal; and the lower limit and the upper limit states are a lower limit current and an upper limit current in the allowable current range.
 15. The reference cell adjusting method of the storage device of claim 14 further comprising the step of controlling the specified voltage bias and an adjustment voltage bias to be applied when adjusting the nonvolatile transistor in response to a bias control signal composed of a plurality of bits.
 16. The reference cell adjusting method of storage device of claim 14 further comprising, when adjusting the current flowing from the nonvolatile transistor when the specified voltage bias is applied in the allowable current range, the steps of: applying an adjustment voltage bias to the nonvolatile transistor; reducing the adjustment voltage bias when the nonvolatile transistor adjusted at the step of applying the adjustment voltage bias is outside the allowable current range as a result of the comparing step; initializing the nonvolatile transistor when the nonvolatile transistor adjusted at the step of applying the adjustment voltage bias is outside the allowable current range as a result of the comparing step; and applying the reduced adjustment voltage bias to the nonvolatile transistor after the initializing step.
 17. The reference cell adjusting method of the storage device of claim 16 wherein the step of applying the adjustment voltage bias and the step of applying the reduced adjustment voltage bias are program operations; and wherein the step of initializing the nonvolatile transistor is an erase operation.
 18. The reference cell adjusting method of the storage device of claim 17 wherein the erase operation is determined to be complete when the current flow from the nonvolatile transistor when the specified voltage bias is applied is greater than the upper limit current; and wherein the program operation is determined to be complete when the current flow from the nonvolatile transistor when the specified voltage bias is applied is between the lower limit current and the upper limit current.
 19. The reference cell adjusting method of the storage device of claim 17 wherein the step of applying the adjustment voltage bias comprises the step of: determining that the nonvolatile transistor adjusted at the step of applying the adjustment voltage bias is outside the allowable current range in response to the current flow from the nonvolatile transistor when the specified voltage bias is applied being smaller than the lower limit current. 